The antigen-antibody reaction is the basis for all immunological test methods. Certain proteins known as antibodies are produced by mammals in response to the presence of an antigen, that is a foreign substance, which can be another protein or a carbohydrate. This normal body response to a foreign substance has led to the development of a number of techniques which are used to diagnose various diseases, disorders and physiological conditions. In a general sense, the component of an immunochemical reaction to be detected is defined herein as the immunoreactive species while the corresponding component reactive with the species is considered the receptor.
For example, in vitro tests for the presence of a suspected protein, antigen or antibody in a biological sample are carried out by adding an immunological counterpart to the biological sample. If the suspected substance is present, the resulting immunochemical reaction can be demonstrated by precipitation of the reaction complex (for example, an antibody-antigen complex). This reaction complex is generally difficult to detect visually. For this reason, either antibodies or antigens are often bound to insoluble particles, for example polymer latex particles, so that when the complex is formed, it is readily detectable from the resulting agglutination by observing either the presence of clumping or a detectable tracer associated with the particles. Agglutination then is characterized by the clumping of particles from a suspension of particles. Further details of known agglutination methods are provided in U.S. Pat. Nos. 4,419,453 (issued Dec. 6, 1983 to Dorman et al) and 4,459,361 (issued July 10, 1984 to Gefter).
Of the several groups of Streptococci, group A Streptococcus (S. pyogenes) is primarily responsible for causing pathological conditions in humans, such as B-hemolytic pneumonia, scarlet fever, rheumatic fever, cardiac sequelae, glomerulonephritis, septic sore throat and puerperal sepsis. Because of the serious nature of infections potentially caused by Streptococcus A, it is important to diagnose its presence in an early stage of infection so that an appropriate course of treatment may be selected. Early tests for detection required culturing a biological sample for long periods of time, usually at least 18 and up to 48 hours. In most cases, such lengthy tests undesirably delay treatment.
More recent agglutination tests for Streptococcus A have been described which are allegedly quicker than the culturing techniques (see, for example, U.S. Pat. No. 4,618,576, issued Oct. 21, 1986 to Rosenstein et al, and E.P. Publication Nos. 150,567 and 174,195).
Nonspecific interactions between latex particles which cause them to agglutinate are normally controlled through electrostatic repulsion. In other words, sufficient similar charges are present on the particles to repel other particles. However, problems have arisen in agglutination assays for a number of immunoreactive species [for example, Streptococcus A, human chorionic gonadotropin (hCG), Chlamydia, Gonorrhea, herpes, HTLV, HIV-I (formerly known as LAV or HTLV-III virus) and others]. When certain proteins (for example, receptors for an immunoreactive species to be determined) are attached to the particles, the resulting net charge is a mixture of positive and negative charges. This leads to significant nonspecific interactions among the particles as well as increased nonspecific interactions between the particles and other charged surfaces in the environment, for example, microporous membranes. It would be desirable to significantly reduce or eliminate all of these nonspecific interactions in order to increase assay sensitivity.